1. Field of the Invention
The present invention relates to a drilling tool endowed with a burr removing function which makes it possible to remove burrs as they are generated on the obverse and reverse sides of a metal plate such as a steel plate, copper plate or aluminum plate when drilling a hole in it.
2. Description of the Related Art
When drilling a hole in a metal plate such as a steel plate with a drilling tool such as a drill, burrs may be generated on the obverse and reverse sides of the metal plate. These burrs make it impossible, for example, to obtain an appropriate clamping force when clamping the metal plate with bolts, rivets or the like. In view of this, a post-processing operation for removing burrs is conventionally performed by using a grinding tool such as a grinder. This burr removing operation requires a lot of effort and time.
To solve this problem, a drilling tool endowed with a burr removing function making it possible to remove burrs as they are generated on the obverse and reverse sides of a workpiece such as a metal plate when drilling a hole in it has been proposed in, for example, Japanese Patent No. 2641694 and Japanese Patent Laid-Open No. 8-155716.
In the drilling tool described in these publications, a cutting edge is provided at the forward end of chip discharging grooves formed in the outer periphery of the tool main body rotated around an axis, and a groove is formed parallel to the axis at the forward end of a land defined between the chip discharging grooves, a burr removing tip held by a cartridge being mounted to this groove such that it can protrude from and retract into the outer peripheral surface of the tool main body. In the above-mentioned cartridge, an arm-like movable portion extending straight parallel to the axis is accommodated in the groove with its rear end portion being supported by the tool main body. The burr removing tip is mounted to the forward end of this movable portion, and the movable portion itself is deformed or rotatably supported by a pin or the like, whereby this burr removing tip can be displaced radially with respect to the tool main body. Further, a resilient force is generated through the deformation of the movable portion, and a plate spring, a compression coil spring or a plurality of belleville springs or the like are provided between the movable portion and the bottom surface of the groove, whereby the burr removing tip is biased toward the outer periphery of the tool.
Further, this burr removing tip is formed substantially as an equilateral-triangular flat plate, with cutting edges being formed on the two sides crossing at the corner portions of the equilateral-triangular surface constituting the scooping surface. This scooping surface is directed in the tool rotating direction, and one of the corner portions is positioned on the outer peripheral side of the tool. The cutting edge crosses the corner portion being mounted so as to angularly protrude from the outer peripheral surface of the tool main body toward the outer peripheral forward end portion of the tool and the outer peripheral rear end portion of the tool. Further, this burr removing tip is mounted to the cartridge such that one corner portion of the equilateral-triangular surface protrudes from the outer periphery of the tool and the bottom side opposed to this corner portion is substantially parallel to the axis of the tool main body as seen from the tool rotating direction.
In this drilling tool, a hole is formed in the workpiece by the cutting edge by causing the tool main body to proceed while rotating, and the peripheral edge of this hole is cut by the cutting edge directed to the outer peripheral forward end of the tool of the burr removing tip, so that it is possible to remove the burrs generated in the opening edge or perform beveling on the peripheral edge of this opening. When the tool main body is caused to further proceed, this burr removing tip is pushed into the inner periphery of the hole of the workpiece to temporarily get within the tool main body, and when the through-hole is formed and the burr removing tip gets out of the hole, it protrudes again from the outer periphery of the tool main body by being biased by the cartridge, so that, by retracting the tool main body from there, it is possible to burr removal, beveling, etc. on the opening peripheral edge of the through-hole.
In this drilling tool endowed with such a burr removing function, the burr removing tip can protrude and retract, so that the abutment of the cutting edge at the time of burr removing operation is unstable, and the sharpness of the cutting edge slightly deteriorates, resulting in a degeneration of the burr removing function. In the drilling tool described in Japanese Patent No. 2641694, the fixation of the burr removing tip in the form of an equilateral-triangular plate is effected by constraining the three sides (the sides connected to the three sides of the triangle) of the burr removing tip with the three holding walls of the cartridge, with the periphery of one apex of the triangle of the burr removing tip protruding and retracting from and into the outer peripheral surface of the tool main body being open. Thus, in the drilling tool disclosed in this publication, the tip cannot be attached or detached without completely drawing the burr removing tip out of the outer peripheral surface of the tool main body by, for example, detaching the cartridge, when replacing the burr removing tip. Thus, the tip cannot be replaced easily. Further, since three holding walls are thus formed in the cartridge, an increase in the size of the cartridge is inevitable.
In the drilling tool described in Japanese Patent Laid-Open No. 8-155716, the two side surfaces of the burr removing tip are constrained by the holding portion of the cartridge to fasten the burr removing tip, so that the tip can be replaced easily. However, in the drilling tool described in this publication, the portion of the holding portion of the cartridge holding the side surface positioned at the tool main body rear end of the burr removing tip is formed by the forward end edge of the strip like member forming the cartridge, so that, in this portion, the side surface of the burr removing tip is simply supported in line contact, and there is a danger of the mounting stability of the burr removing tip being impaired. In particular, this portion holding the side surface positioned at the tool main body rear end of the burr removing tip is the portion which receives the load applied to the burr removing tip when drilling by the drilling tool, so that, if the portion can only hold the burr removing tip in line contact, there is the danger of the burrs not being reliably removed.
Further, in the drilling tool disclosed in the above publication, the biasing member is a plate spring, a compression coil spring or a belleville spring as stated above, so that when it is necessary to increase the biasing force of this biasing member according to, for example, the machining condition, it is necessary to increase the thickness of the plate spring, increase the number of turns or the thickness of the compression coil spring, or increase the number of belleville springs. In any case, an increase in the distance between the cartridge movable portion and the bottom surface of the groove is inevitable. As a result, the groove accommodating the cartridge becomes deeper, and the tool main body is greatly cut out, so that, in particular, in a drilling tool for drilling small diameter holes, the rigidity of the tool is impaired and breakage, etc. occurs, or it may become impossible to form a groove capable of accommodating such a cartridge.
Further, in the above drilling tool, the burr removing tip in the form of a substantially equilateral-triangular flat plate is arranged so as to be parallel to the axis of the tool main body as seen from the tool rotating direction, with its triangular surface being directed toward the tool rotating direction as the scooping surface, and one corner portion of the triangular surface protruding toward the outer periphery of the tool, and the bottom side of the triangular surface opposed to this one corner portion protruding toward the outer periphery of the tool, that is, in the condition in which the burr removing tip has not been forced into the hole of the workpiece yet. However, when the burr removing tip is displaced toward the inner periphery of the tool by deforming the movable portion of the cartridge or rotating it around the pin as the burr removing tip is forced into the hole of the workpiece, the retraction into the groove is effected with the burr removing tip inclining so as to incline toward the inner periphery of the tool as the bottom side is changed from the position parallel to the axis toward the forward end of the tool. Thus, when getting the burr removing tip into the groove with the bottom side being thus inclined, it is necessary for the depth of the groove to be not less than the distance from the bottom side of the equilateral triangular surface to one corner portion, and, in the case of a small-diameter drilling tool, the rigidity of the tool is impaired or it is difficult to form the groove itself.
Further, in the above drilling tool, the movable portion of the cartridge is arranged so as to extend parallel to the axis of the tool main body, and, as a result, the groove accommodating the cartridge is also formed so as to extend parallel to the axis of the tool main body. However, when the chip discharge groove is twisted around the axis of the tool main body as in a twist drill, the land of the tool main body in which the groove is formed is similarly twisted around the axis. In particular, in a small diameter drilling tool or a drilling tool in which the twisting angle of the chip discharge groove is large as described above, the axial length of the land is also small. Thus, there is a limitation to the formation of a groove to extend parallel to the axis of the tool main body as described above, and it has been impossible to attach to such a drilling tool a cartridge of which the length of the movable portion is large.